Field of the Invention
The present invention relates to a condenser microphone having a plurality of condenser microphone units connected in series to improve output sensitivity.
Description of the Related Art
A condenser microphone generates an audio signal based on a change in capacitance between a diaphragm and a fixed electrode which are opposed to each other.
In this configuration, a condenser microphone unit is disposed with the diaphragm opposing to the fixed electrode, and the condenser microphone unit has a capacitance of approximately several ten picofarads, and an output impedance is considerably high. Therefore, the audio signal is extracted using an impedance converter using for example a field effect transistor (FET).
Various devices for improving the output sensitivity of the condenser microphone have been proposed conventionally, a condenser microphone using a plurality of condenser microphone units connected in series to improve output sensitivity has been proposed by an applicant of the present invention, and disclosed in JP 5201598 B2.
FIG. 4 is a block diagram illustrating a configuration of a condenser microphone disclosed in JP 5201598 B2. In the configuration illustrated in FIG. 4, the condenser microphone unit is formed that the diaphragm is opposed to the fixed electrode, and audio signals obtained from first and second condenser microphone units U1 and U2 are added in series in the same phase.
That is, the diaphragm 1a of the first condenser microphone unit U1 is grounded, and the fixed electrode 2a opposed to the diaphragm 1a is connected to the first impedance converter 3a. Therefore, the audio signal is generated by the impedance converter 3a based on a change in capacitance between the diaphragm 1a and the fixed electrode 2a of the condenser microphone unit U1.
As described below, the impedance converter 3a constitutes a source follower circuit using an active element such as the FET, and an operation current is supplied from a DC power supply Vcc to the impedance converter 3a. 
The first impedance converter 3a generates the audio signal of a first condenser microphone unit U1, and the audio signal is configured to drive a reference potential point of the second condenser microphone unit U2. That is, in an example illustrated in FIG. 4, output from the first impedance converter 3a is supplied to the diaphragm 1b constituting the second condenser microphone unit U2.
The fixed electrode 2b opposed to the diaphragm 1b is connected to the second impedance converter 3b, and the audio signal is generated by the impedance converter 3b based on a change in capacitance between the diaphragm 1b and the fixed electrode 2b of the condenser microphone unit U2.
In this configuration, in the second condenser microphone unit U2, the audio signal obtained from the first condenser microphone unit U1 is applied to the reference potential point (diaphragm 1b) of the first condenser microphone unit U1. Accordingly, the audio signals obtained from the first and second condenser microphone units U1 and U2 are added in series in the same phase, are output from an output terminal Out of the second impedance converter 3b. 
It is noted that also in the second impedance converter 3b, the source follower circuit is constituted for example using the FET, similar to the first impedance converter 3a. 
According to a configuration of the condenser microphone illustrated in FIG. 4, the audio signals obtained from the first and second condenser microphone units U1 and U2 are added in phase, and a resultant signal is output. Therefore, the output sensitivity as the condenser microphone can be doubled.
In addition to the example illustrated in FIG. 4, FIG. 5 represents an example of a condenser microphone including a third condenser microphone unit U3, and the third condenser microphone unit U3 includes a third impedance converter 3c similar to the impedance converters having been described above.
According to a configuration illustrated in FIG. 5, audio signals obtained from the first to third condenser microphone units U1 to U3 are added in phase and a resultant signal is output. Therefore, the output sensitivity as the condenser microphone can be further increased.
In the condenser microphone disclosed in JP 5201598 B2, the impedance converters need to be provided corresponding to the plurality of condenser microphone units connected in series, respectively, as described above.
Each of the impedance converters employs the active element such as FET. Therefore, each of the impedance converters requires a circuit configuration for driving the active element, and further requires a configuration for supplying operation power (DC power supply Vcc) for the active element. Therefore, disadvantageously, the whole circuit structure of the condenser microphone is complicated, and inevitably results in a high cost.
Further, the operation power supply uses a limited power supply such as a known phantom power supply or a battery, and when a plurality of impedance converters using active elements such as an FET is used, each of the impedance converters requires a drive current. As a result, even if for example the phantom power supply is used, the condenser microphone has such a problem that a drop in voltage of the operation power supply for the impedance converter is increased, a maximum output level of the condenser microphone is also limited, and it is difficult to increase a dynamic range.
The present invention has been made in view of the above-mentioned problems of a conventional condenser microphone including the impedance converter using the active element for each of the condenser microphone units, as described above, and it was found that preceding condenser microphone units are directly connected, excepting an impedance converter connected to a last condenser microphone unit, so that an audio signal obtained from a preceding condenser microphone unit is added to an audio signal obtained from a succeeding condenser microphone unit and transmitted.